In olefin epoxidation an olefin is reacted with oxygen to form an olefin epoxide, using a catalyst comprising a silver component, usually with one or more further elements deposited therewith on a support. The olefin oxide may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine.
The performance of the epoxidation process may be assessed on the basis of the selectivity, the catalyst's activity and stability of operation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. Modern silver-based epoxidation catalysts are highly selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above 85.7 mole-% at start of cycle and under favorable conditions (e.g., low work rate, low delta EO and low CO2). An example of such highly selective catalysts is a catalyst comprising silver and a rhenium promoter, for example U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105.
Many process improvements are known that can improve selectivity. See, e.g., U.S. Pat. No. 7,235,677; U.S. Pat. No. 7,193,094; US Pub. Pat. App. 2007/0129557; WO 2004/078736; WO 2004/078737; and EP 2,155,708. These patents also disclose that water concentration in the reactor feed should be maintained at a level of at most 0.35 mole percent, preferably less than 0.2 mole percent. Other patents disclose control of the chloride moderator to maintain good activity. See, e.g., U.S. Pat. No. 7,657,331; EP 1,458,698; and U.S. Pub. Pat. App. 2009/0069583. Still further, there are many other patents dealing with EO process operation and means to improve the performance of the catalyst in the process. See, e.g., U.S. Pat. Nos. 7,485,597, 7,102,022, 6,717,001, 7,348,444, and U.S. Pub. Pat. App. 2009/0234144.
All catalysts must first be started up in a manner to first establish a good selectivity operation. U.S. Pat. No. 7,102,022 relates to the start-up of an epoxidation process wherein a highly selective catalyst is employed. In this patent there is disclosed an improved start-up procedure wherein the highly selective catalyst is subjected to a heat treatment wherein the catalyst is contacted with a feed comprising oxygen at a temperature above the normal operating temperature of the highly selective catalyst (i.e., above 260° C.). U.S. Pub. Pat. App. 2004/0049061 relates to a method of improving the selectivity of a highly selective catalyst having a low silver density. In this document, there is disclosed a method wherein the highly selective catalyst is subjected to a heat treatment which comprises contacting the catalyst with a feed comprising oxygen at a temperature above the normal operating temperature of the highly selective catalyst (i.e., above 250° C.). U.S. Pat. No. 4,874,879 relates to the start-up of an epoxidation process employing a highly selective catalyst wherein the highly selective catalyst is first contacted with a feed containing an organic chloride moderator and ethylene, and optionally a ballast gas, at a temperature below the normal operating temperature of the catalyst. EP-B1-1532125 relates to an improved start-up procedure wherein the highly selective catalyst is first subjected to a pre-soak phase in the presence of a feed containing an organic halide and is then subjected to a stripping phase in the presence of a feed which is free of the organic halide or may comprise the organic halide in a low quantity. The stripping phase is taught to continue for a period of more than 16 hours up to 200 hours. U.S. Pat. App. No. 2009/0281339 relates to the start-up where the organic chloride in the feed is adjusted to a value sufficient to produce EO at a substantially optimum selectivity. U.S. Pat. No. 7,553,980 teaches a process for initiating a highly selective ethylene oxide catalyst in which the highly selective ethylene oxide catalyst is operated first as a ‘standard’ Ag-based catalyst (e.g., a catalyst that contains only silver and alkali metal, especially cesium). Moreover, the inventive initiation procedure is more efficient when the concentration of carbon dioxide in the feed is higher than 6 vol. %, and even more efficient when the concentration of carbon dioxide in the feed is higher than 10 vol. %, of the feed mixture during the initiation period.
At the end of the start-up period, the operating conditions of the plant are set to their “normal” conditions. Work rate is set to meet the production demands of the plant. The space velocity is typically set by operating the recycle gas compressor at its maximum throughput, although a lower space velocity is sometimes used to save on the cost of electricity to run the compressor. The unit pressure is usually set by the unit design and is rarely changed. The ethylene level is typically set via design constraints, the presence or absence of an ethylene recovery unit, and economic considerations. Most plants utilize some type of “flammability equation” which defines the maximum safe level of oxygen at the reactor inlet (the “flammable limit”). Based on safety concerns or past experience, the plant will define some “flammable margin” which defines how high their oxygen level is allowed to be. For example, if a plant determined that the flammable limit is 8.5%, and for safety reasons they desire a flammable margin of 0.5%, then they will operate the plant with an inlet oxygen of 8.0% or lower. Typically, a plant will operate their oxygen level as high as their flammable margin restrictions will allow. Finally, with a high-selectivity catalyst, the chloride level will be set to maximize selectivity, and it will be changed throughout life to maintain operation at a level which maximizes selectivity.
CO2 levels are always present in the feed of an EO reactor, with the actual level being a function of the amount of CO2 production over the catalyst, the size of the CO2 absorber, and the extent to which the CO2 absorber is being used. The CO2 removal system is typically run as hard as possible, from the end of the start-phase through end of life, in order to minimize the CO2 level at the reactor inlet. This is done because it is recognized that lower CO2 levels always lead to lower reaction temperatures (at a constant production rate). Lower temperatures are commonly believed to increase the catalyst life and to improve the selectivity. These same beliefs have led plant design in recent years to have both low CO2 levels and low production rates, which are the two factors that most influence reaction temperature. In the past it was common for the feed CO2 level in a plant using high-selectivity catalyst to be 3% or higher; currently many EO plants operate at below 1% CO2, with some plants operating below 0.3% (pushing the lower limit).